PBI (Polybenzimidazole), a versatile material of thermo-chemically and mechanically stable, acid doped PBI is widely demonstrated as a polyelectrolyte membrane material for High Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC), but still its conductivity needs to be improved.
The acid doping behavior of polybenzimidazole membranes in phosphoric acid for proton exchange membrane fuel cells is reported in Fibers and Polymers 9, (6), pp 679-684, 2008 by Ronghuan et al.
CN102945977 discloses a composite proton exchange membrane doped with zwitter-ion modified graphene oxide for a methanol fuel cell and a method for preparation thereof.
US2012270122 relates to a method of operating a fuel cell system with a proton exchange membrane, wherein the membrane is composed of a polymeric material comprising acid-doped polybenzimidazole (PBI).
Further, the composite membranes of PBI and ionic liquid (IL) are reported in the art to exhibit high proton conductivity at high temperatures. However, the addition of IL to PBI decreased mechanical properties of resulting PBI-IL, further the ionic liquids tend to leach out from the membrane after prolonged usage.
The PILs are anticipated to provide a continuous pathway of IL character (which is present on the repeat unit of the polymer) and also eliminate the issue of IL drain. Moreover, PILs exhibit the properties of IL such as ionic conductivity, thermal stability, tunable properties and chemical stability.
Research is now focussed to improve the proton conductivity of the membranes by incorporation of polymeric ionic liquids into PBI solution and formation of PBI-PIL blend membranes.
Research is now focussed to improve the proton conductivity of the membranes by incorporating different composites into PBI.
PIL is a unique polymer that can be used for proton conduction by blending with PBI which causes it to retain its inherent characteristics such as ionic property in backbone and high solvent solubility. Thus proton conductivity of blend membranes can be elevated considerably.
The present inventors sought to provide an alternate PIL selected from Poly (diallyl dimethyl ammonium) trifluoromethane sulphonate for improving physical and electrochemical properties and for applicability for high temperature PEMFC.
The present inventors sought to provide a PIL, poly(diallyl dimethyl ammonium) trifluoromethane sulphonate to blend with PBI for improving physical and electrochemical properties for high temperature PEMFC.
The main objective of the present invention is to provide blend membranes based on polybenzimidazole (PBI) and polymeric ionic liquids (PILs) and a process for the preparation thereof.
Accordingly, the present invention provides stable blend membrane comprising of polybenzimidazole (PBI) and polymeric ionic liquid (PIL) Poly (diallyl dimethyl ammonium) trifluoromethane sulphonate P[PDADMA][TFMS] with enhanced proton and hydroxyl ion conductivity.
In an embodiment of the present invention the weight ratio of PBI-I:PIL in the blend membrane is selected from 95:5, 85:15, 75:25, 65:35 and 55:45.
In one embodiment of the present invention said blend membrane is doped with phosphoric acid with concentration in the range of 1-17M.
In another embodiment of the present invention said blend membrane has a thickness in the range 25-300 μm.
Still in another embodiment of the present invention a process for the preparation of blend membrane of PBI and P[PDADMA][TFMS] wherein the said process comprises adding a solution of P[DADMA][TFMS] in DMSO to a solution of PBI-I in DMAc (Dimethylacetamide) at room temperature with stirring until complete mixing for 11-13 h subsequently casting the mixed solution on a flat glass surface to obtain dense blend membranes and drying in vacuum at a temperature of about 80° C. for about 8 days to remove residual solvent and doping the blend membrane with phosphoric acid.
Still in another embodiment of the present invention P[DADMA][TFMS] is prepared using anion exchange of P[DADMA][Cl] comprising adding equimolar quantity of silver salt to about 8% solution of P[DADMA][Cl] prepared in water and stirring at ambient temperature to ensure maximum possible exchange further centrifuging to separate AgCl followed by evaporation of the supernatant solution to obtain PIL, P[DADMA][TFMS].
Still in another embodiment of the present invention the silver salt is selected from metal salt of carboxylate, sulfonate, halogen, NO3−, NO2−, PO3−, BF4−, HPO4—N(SO2CF3)−2, H2PO4−, HSO4−, S2O3−, ClO−, BrO3−, CrO4−, HCO3−, C2O4−, MnO4−, NH2−, FeCl4−, PF6−, (CN)2N−, C16H34PO4−, C12H25C6H4O3−, SCN−, CH2═CHCOOCH2, CH2CH2SO3−, CH2═CHSO3−, and C6H4CO3SN−.
Still in another embodiment of the present invention proton and hydroxyl ion conductivity of stable blend membrane is in the range 0.03-0.08 and 0.04-0.14 Scm−1 respectively.
Still in another embodiment of the present invention stable blend membrane of PBI-I and P[PDADMA][TFMS] is useful as membrane electrolyte for high temperature PEMFC.
The present invention provides mechanically and thermally stable blend membrane comprising of polybenzimidazole (PBI) and polymeric ionic liquid (PIL) Poly(diallyldimethylammonium)trifluoromethane sulphonate, in varying ratios, with enhanced proton and hydroxyl ions conductivity. The weight ratio of PBI-I:PIL in the blend membrane is selected from 95:5, 85:15, 75:25, 65:35 and 55:45.
The excellent mechanical stability of the membrane is one of the important requirements for applicability in high temperature PEMFC. In an aspect, the blend membranes of the instant invention are doped with phosphoric acid with concentration ranging from 1-17 M. The formed blend membranes are observed to be stable to doping with phosphoric acid of concentration 15M.
In another aspect, the invention provides preparation of polybenzimidazole which comprises polycondensation reaction of 3,3′-diaminobenzidine (DAB) and isophthalic acid, in presence of polyphosphoric acid (PPA) at elevated temperature of 120-250° C. as reported earlier in the art.
In an aspect, the present invention provides a process for preparation of Poly (diallyl dimethyl ammonium) trifluoromethane sulphonate based on aliphatic backbone using anion exchange of poly(diallyldimethylammonium chloride), P[DADMA][Cl] in suitable concentration. The metal salt for anion exchange is selected from metal salt of methyl sulphonate, trifluoro methyl sulphonate and p-toluene sulphonate and other salts. The metal halide is separated by centrifugation or other known techniques. The desired polymeric ionic liquid is obtained by evaporation of the supernatant solution (cf scheme 2).
Additionally, the anion exchange salt may be selected from the metal salt of carboxylate, sulfonate, halogen, NO3−, NO2−, PO3−, BF4−, HPO4—N(SO2CF3)−2, H2PO4−, HSO4−, S2O3−, ClO−, BrO3−, CrO4−, HCO3−, C2O4−, MnO4−, NH2−, FeCl4−, PF6−, (CN)2N−, C16H34PO4−, C12H25C6H4O3−, SCN−, CH2═CHCOOCH2, CH2CH2SO3−, CH2═CHSO3−, and C6H4CO3SN−.
The anion exchange of PIL is evaluated by estimation of chloride content in formed PIL by known Volhard's method [G. H. Jeffery, J. Bassett, J. Mendham and C. Denney, Vogel's
Textbook of Quantitative Chemical Analysis, British Library Cataloguing in Publication Data, 5th edn, 1989, pp. 355-356.].
In another aspect, the present invention provides a process for preparation of said blend membrane. The process includes adding a solution of P[DADMA][TFMS] dissolved in DMSO to a solution of PBI-I dissolved in DMAc (3% solution) at room temperature with stirring until complete mixing; casting the mixed solution on a flat glass surface to obtain dense blend membranes and drying in vacuum for about 8 days to remove residual solvent. The blend membrane is further doped with phosphoric acid. The thickness of the blend membrane is in the range of 25-300 μm.
In yet another aspect, the present invention provides the blend membrane of PBI-I and Poly (diallyl dimethyl ammonium) trifluoromethane sulphonate which show increase in proton conduction in comparison to the PBI membranes alone due to the presence of ionic groups of PILs in blend membranes. Single cell were successfully tested with blend membranes at 160° C.; the obtained maximum power density and current density were also higher than the pristine PBI-I membrane.
In another aspect the invention present invention provides polybenzimidazole (PBI) having formula I,

In another aspect the invention present invention provides polymeric ionic liquids (PILs) having formula IV,

wherein, the substituent are represented in detailed description herein below.